A semiconductor photodiode in general includes a substrate of semiconductor material having at least one epitaxial layer of semiconductor material on a surface thereof in which is included adjacent regions of opposite conductivity type forming a PN junction or an intrinsic region between spaced regions of opposite conductivity type forming a PIN device. These regions of opposite conductivity type may be formed by successive growth of appropriately doped epitaxial layer or by other techniques, such as ion implantation or diffusion. Also, the particular semiconductor material used is dependent on the wavelength of the light to be detected. Recently, photodiodes having one or more epitaxial layers of indium gallium arsenide (InGaAs) on a substrate of indium phosphide (InP) have come into use for detecting light in the wavelength range of approximately 1 to 1.6 micrometers.
In one type of photodiode, the light being detected is directed into the diode through the topmost layer. This type of photodiode generally includes a layer of insulating material on the surface of the topmost layer. In a photodiode which is of a planar geometry, the insulating layer serves the purpose of passivating the diode, and may also serve as an antireflection coating for light entering from this surface. The insulating layer has an opening therethrough through which a conductive layer extends to make contact with a portion of the topmost layer. The contact may also extend over the insulating layer in the form of a pad to which a terminal can be attached. A problem with this type of photodiode is that the additional diode area required for electrical contact plus the contact pad on the upper surface of the insulating layer provides additional capacitance which adversely affects the sensitivity of the photodiode in certain amplifier configurations.
In another type of photodiode, the light being detected is directed into the diode through the substrate. In this type of photodiode the conductive contact for the topmost layer can be applied over the whole area of the diode to serve as the contact pad. This reduces the capacitance of the device. However, this type of device has a problem in that the substrate is generally highly doped so as to have a low resistance, and the highly doped material is absorbtive of the light so that not all the light reaches the active layer.